1. Field of the Invention
This invention generally relates to a data communication system, and, in particular, to a data communication system having an ARQ (Automatic Request for Repetition) function. More specifically, the present invention relates to an ARQ equipped data communication system capable of rapidly shifting down the data transmission rate when using a transmission line of poor quality.
2. Description of the Prior Art
In general, in a data communication system, in order to transmit data securely from a transmitter to a receiver through a transmission line, use is commonly made of an Automatic Request for Repetition (hereinafter simply referred to as "ARQ") system, in which a Cyclic Redundancy Checkcord, or simply "CRC", bit to be used for detecting the presence of an error is added to the information to be transmitted as input from a terminal device into the transmitter and such CRC bit added information is converted into codes, which are then transmitted to the receiver block by block, each block consisting of a predetermined number of characters, the receiver transmitting a signal requesting retransmission of a block if an error has been detected in the corresponding block.
For example, in the hand-shaking system, which falls onto the category of the ARQ system, on the basis of a backward signal transmitted from the receiver in half-duplex communication, the transmitter determines whether to transmit the next following block of new data or to retransmit the last preceding block of old data. On the other hand, the receiver examines whether a block of data transmitted from the transmitter has been received without error and then supplies a backward signal on the basis of its result. In accordance with another system which is directed to shorten the data transmission time period, the transmitter transmits a plurality of blocks of data consecutively; on the other hand, upon receipt of these blocks of data, the receiver carries out error detection for each of the blocks of data thus received and then supplies the block number of the block having an error to the transmitter. In a single block of data, in the case of a facsimile communication system, there are contained image information, flag command (including block number) and CRC bit, so that the receiver examines the CRC bit of the block received and compares its result with the added CRC bit, whereby the receiver requests retransmission of block of data to the transmitter if they do not match, or the block number received differs from the expected number, or the flag has been destroyed.
Under the circumstances, due to various reasons such as machine failure and failure or abnormal conditions of transmission line, error may be detected at the receiver even if the same block of data is repetitively transmitted over a number of times. In such a case, in accordance with the conventional ARQ data communication system, if the number of retransmission trials exceeds a predetermined number, the hung-up condition is established thereby supplying a disconnect ("DCN") command and having the transmission line disconnected. However, it is often the case that such a disconnected transmission could be carried out completely without error and hung-up if it were carried out at a lower data transmission rate. Nonetheless, since the conventional ARQ data communication system is not equipped with a function of shifting down the data transmission rate during transmission of image information, the data transmission method described above cannot be adopted.
Under the foregoing premise, a patent application has been filed (Japanese Patent Appln. No. 56-171055, assigned to the assignee of this U.S. Patent application) on an invention of data communication system having a shift down function, wherein, in an ARQ equipped facsimile communication system, upon detection of the condition that the number of retransmission trials has exceeded a predetermined number at the transmitter, a shift down command is supplied to the receiver using a slower MODEM different from the MODEM which has so far been used in transmitting image information, thereby allowing to transmit image information at a shifted down data transmission rate. With such a system, it is now possible to shift down the data transmission rate during the transmission of image information, so that hung-up condition can be avoided even if retransmission is requested over a predetermined number of times. In addition, since a shift down command is supplied at a slower MODEM which differs from the MODEM used in transmission of image information, the command can be supplied to the receiver securely.
However, transmitters and receivers are installed at quite a variety of places so that the quality of transmission line also varies markedly from place to place and it may be quite poor at some places. There are also various causes to make the transmission line low in quality; for example, in some places, the transmission line may be always low in quality because it has a lower S/N ratio, or, in other places, it may be occasionally low in quality because of route and other environmental conditions. For this reason, in the case of transmitting image information in the above-described data communication system having a shift down function by shifting down the data transmission rate automatically whenever a predetermined condition is met at the transmitter, i.e., the number of request for retransmission having exceeded a predetermined number, the communication of image information between the two terminals both at the places where the quality of transmission line is poor will take an exorbitantly long period of time because it takes time until a proper MODEM has been found.
For example, in the case where four steps of MODEM rate, i.e., 9,600 bps, 7,200 bps, 4,800 bps and 2,400 bps are available, if the transmitter shifts down the data transmission rate one step each time upon receipt of the retransmission request 32 times from the receiver, the time period T necessary for shifting down from 9,600 bps to 2,400 bps may be calculated in the following manner, assuming that the data of approximately 4 k bits are to be transmitted during the period of single retransmission. EQU T=4,000.times.32.times.(1/9,600+1/7,200+1/4,800) .congruent.59 sec.
In this manner, it takes approximately one minute to reach an appropriate MODEM rate, so that the data transmission period tends to be prolonged thereby lowering the use rate of transmission line.